The present invention generally relates to a vehicle transmission control system and, more particularly, to a system and method for controlling double transition shifts in transmissions which utilize multiple gearing sections.
Motor vehicles require a transmission between the vehicle engine or motor and the output drive elements in order to optimize efficiency and provide the desired torque and acceleration characteristics under varying driving conditions while maintaining the engine or motor within operational speeds. A typical transmission includes a number of individual gear elements which may be selectively engaged using a corresponding set of individual clutches. The combination of activated clutches determines the overall speed ratio between the input and output shafts of the transmission. In a simple transmission, a shift from a current speed ratio to a new speed ratio involves disengaging a first clutch, known as the off-going clutch, and engaging a second clutch, known as the on-coming clutch. In certain applications, however, the transmission may comprise multiple gearing sections with an intermediate shaft therebetween in order to optimize manufacturing costs, size, or other operational parameters.
The use of multiple gearing sections may result in at least one instance where multiple clutches in the transmission are being engaged or disengaged at the same time in order to achieve a desired change in the overall speed ratio of the transmission. For example, in a ten-speed transmission comprising a five-speed range pack followed by a two-speed splitter unit, the shift from fifth to sixth gear involves the coordination of four clutches: the off-going and on-coming clutches in the range pack, as well as the off-going and on-coming clutches in the splitter unit. These shifts, commonly referred to as double transition shifts, are more difficult to control due to the complex interactions between the simultaneous shifts in the range pack and splitter unit. Furthermore, a double transition shift often requires that one transmission section (e.g., the range pack) be shifted from its highest gear ratio to its lowest gear ratio, which can result in excess heat buildup in the clutch elements. One known method for dealing with the problem is to simply speed up the rate of the shift, thereby reducing the time available for heat to build up in the clutches.
However, many transmission control systems utilize what is known as “power on shifting” where there is very little, if any, decrease in delivered output torque during a transmission shift and the shifts are completed in a shorter amount of time. This increases the efficiency and responsiveness of the transmission, but also makes control of the various clutches and other transmission elements more difficult, particularly in the case of double transition shifts. Systems and methods are therefore needed which improve shift quality and reduce strain on the transmission elements during double transitions shifts.